Harrison Nicholls / Curriculum Vitae

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My research explains observed exoplanetary properties and atmospheric compositions, while making predictions as to their histories and formation scenarios. This is undertaken through numerical modelling, which captures the physics active from planet formation up to the present day.

Employment and education

Research Associate in Exoplanet Modelling

University of Cambridge, United Kingdom
Feburary 2026 — Ongoing

Postdoctoral research associate working with Oli Shorttle at the Institute of Astronomy. Investigating the role of volatile chemistry, solubility, and gas radiative properties in controlling the observable characteristics of sub-Neptune/super-Earth exoplanets.

DPhil in Atmospheric Oceanic and Planetary Physics

University of Oxford, United Kingdom
October 2022 — January 2026

I developed numerical models to explore the evolution of rocky (exo)planets, providing new perspectives on how coupled interior-atmosphere interactions and their feedbacks shape planets over deep time. The PROTEUS framework has now become an established community modelling tool. Supervised by Ray Pierrehumbert and co-supervised by Tim Lichtenberg. Examined by Paolo Sossi and Tad Komacek, and awarded without corrections, my thesis is available below.

MPhys in Physics

University of Exeter, United Kingdom
September 2018 — July 2022

Four-year integrated MPhys Physics programme completed with a 1st (final grade of 85%). Modules included: astrophysics, electromagnetism, analytical dynamics, thermodynamics, scientific programming, and extensive partnered lab work. Supervised by Éric Hébrard; my dissertation was published in MNRAS as Nicholls+23.

Scientific publications

My first-author publications are outlined below.
A complete list of publications is available on NASA-ADS / SciX.

Lifetimes in lineland: Rocky planets are one-dimensional during their early evolution

Zhan, Nicholls, Shorttle, Koll (in prep)
In prep for ApJ

As joint first authors on a project I proposed and supervised, we consider whether rocky planets can be treated as one-dimensional during their early evolution. Using 1D planet evolution calculations and 3D global climate simulations, we find thatthat 1D models are sufficient to capture the key physics of rocky planet evolution.

2026

Constraining the lives and times of exoplanets through evolutionary Bayesian retrievals

Nicholls, Lichtenberg, Riegler, Calder, and Fortuin (2026e)
In review at ApJ, arxiv:2607.25845, pdf:2607.25845.pdf

Can the lifetime histories of exoplanets' interiors and atmospheres be recovered from present-day observations, despite known degeneracies? I test the efficacy of evolutionary parameter-inference, by implementing PROTEUS' forward models within a new machine learning Bayesian optimisation algorithm. I constrain mantle redox, core fraction, and volatile inventories; recovering post-formation volatile budgets to less than 20 pct. error for Earth-sized planets.

CHILI Planetary Evolution Intercomparison — Primordial Magma Oceans of Earth and Venus

Nicholls, Krissansen-Totton, Lichtenberg, Schaefer, et al. (2026d)
In review at PSJ, arxiv:2606.24757, pdf:2606.24757.pdf

Part I of the CHILI project, part of the NASA CUISINES framework, which intercompares planetary evolution models by adopting Earth and Venus' earliest histories as case studies. Models disagree by their parametrisations of volatile partitioning and radiative transfer. Meanwhile, their agreement suggests that Earth could have supported multiple "magma ocean" periods before its final solidification.

Beyond the mass-radius plane: simulations of the exoplanet continuum

Nicholls, Shorttle, Lichtenberg, Pascal (2026c)
Published in MNRAS, doi:10.1093/mnras/stag1489, arxiv:2604.15891, pdf:2604.15891.pdf

I develop a framework for recovering exoplanet properties which unifies deep planetary interiors with radiative-convective-chemical climate calculations. Envelope mass fractions are degenerate with instellation flux and atmospheric metallicity; overlooked in previous studies. Habitable zone sub-Neptunes have supercritical, uninhabitable surfaces. I introduce a Bayesian retrieval tool, moving us toward a comprehensive picture of the exoplanet continuum.

Volatile-rich evolution of molten super-Earth L 98-59 d

Nicholls, Lichtenberg, Chatterjee, Guimond, Pierrehumbert (2026b)
Published in Nature Astronomy, doi:10.1038/s41550-026-02815-8, arxiv:2507.02656, pdf:2507.02656.pdf

I explain the formation and lifetime evolution of super-Earth L 98-59 d using PROTEUS. My models reproduce JWST observations of its hybrid H2-H2S-SO2 atmosphere. These results show that it formed volatile-rich and geochemically-reducing, inviting a more nuanced classification framework for small planets beyond the 'water world' and 'gas dwarf' paradigms. Presented at Exoclimes VII, EPSC-DPS, UKExoM, and Exoplanets 6.

Our discovery of a potential magma ocean 'sulfur world' gained coverage in The Guardian, Reuters, the Financial Times, Scientific American, and other outlets. I also discussed this research and its implications on the CBC 'Quirks' radio programme.

What happened to rocky planets?

Nicholls (2026a), DPhil Thesis
University of Oxford, doi:10.5287/ora-bmz5xpbrk, pdf:thesis.pdf

To explore the question of why rocky planets (in the Solar System and beyond) are observed to span such a wide diversity of climate states, I developed and applied the PROTEUS framework to directly simulate the multi-physical evolution of planets over their billion year histories. I highlight the key processes active during early magma ocean periods, including novel atmosphere-interior feedbacks and key climate physics. Applying my modelling to specific exoplanets bridges the gap between observational constraints from JWST and the hidden atmospheric/geological mechanisms which shape rocky worlds across the universe.

2025

Self-limited tidal heating and prolonged magma oceans in the L 98-59 system

Nicholls, Guimond, Hay, Chatterjee, Lichtenberg, Pierrehumbert (2025c)
Published in MNRAS, doi:10.1093/mnras/staf1167, arxiv:2505.03604, pdf:2505.03604.pdf

Observations of L 98-59 indicate that it hosts three planets with large orbital eccentricities. I calculate the tidal heating within these planets self-consistently with outgassing and atmospheric energy transport. A robust feedback between tidal heating, radiative cooling, and mantle rheology leads to self-regulation of tidal heating. Tidal heating may allow them remain molten for a long time, and raises the potential for 'tidally supported worlds'.

AGNI: A radiative-convective model for lava planet atmospheres

Nicholls, Pierrehumbert, Lichtenberg (2025b)
Published in JOSS, doi:10.21105/joss.07726, arxiv:2506.00091, pdf:2506.00091.pdf

Accurate models for the atmospheres of lava planets are critical for understanding their evolution and present states. I present a new radiative-convective real-gas atmosphere model for lava planets. This model is designed to be flexible and accurate, allowing for rapid exploration of parameter spaces while including a suite of physical processes.

Convective shutdown in the atmospheres of lava worlds

Nicholls, Pierrehumbert, Lichtenberg, Soucasse, Smeets (2025a)
Published in MNRAS, doi:10.1093/mnras/stae2772, arxiv:2412.11987, pdf:2412.11987.pdf

Using a new radiative-convective atmosphere model 'AGNI', I found that atmospheres on permanent lava worlds can be convectively stable. Absorption features of CO2 and SO2 within emission spectra are associated with mantle redox state. Presented at UKPF Meeting 2025.

2024

Magma Ocean Evolution at Arbitrary Redox State

Nicholls, Lichtenberg, Bower, Pierrehumbert (2024)
Published in JGR Planets, doi:10.1029/2024JE008576, arxiv:2411.19137, pdf:2411.19137.pdf

With a new interior-atmosphere modelling framework 'PROTEUS', I simulated the evolution and cooling of primordial terrestrial magma oceans. The greenhouse effect of outgassed atmospheres exerts significant control over magma ocean cooling and crystallisation. Presented at EGU.

2023

Self-Consistent Modelling of Flares and Gas Giants

Nicholls, Hebrard, Venot, Drummond, Evans (2023)
Published in MNRAS, doi:10.1093/mnras/stad1734, arxiv:2306.03673, pdf:2306.03673.pdf

Using a radiative-convective photochemical kinetics model, I found that flares induce observable features in the spectra of sufficiently cool gas giants orbiting M-stars. Presented at Exoclimes VI.

Conferences and workshops

Teaching and academic service

Supervision of PhD student summer collaboration

Summer 2026, Cambridge
With Ruizhi Zhan, we proposed and obtained funding for a summer research project to explore the role of 3D climate physics in shaping the evolution of rocky exoplanets. We obtained funding from Peking University for her to be supervised at the Institute of Astronomy, University of Cambridge. This research is in-prep for submission to ApJ.

Rocky Worlds Discussions host

Summer 2026 — Ongoing, Cambridge
I co-host the established Rocky Worlds Discussions seminar series, and contribute to its organisation and management. RWD is a monthly seminar which bridges planetary science, exoplanet astronomy, and geophysics. It is associated with the Rocky Worlds conferences.

Internal examiner for PhD student progress

Summer 2026, Cambridge
I have served as an internal examiner assessing PhD student progress at the Institute of Astronomy, University of Cambridge.

Journal club chair and seminar organiser

Winter 2024 — Spring 2026, Oxford
I had the pleasure to chair and organise the Oxoplanets journal club and seminar series within the Department of Physics, University of Oxford. This work involved inviting speakers, organising schedule, and chairing ~30 person-strong science discussions every week.

Reviewer for scientific journals and UK research councils

Winter 2024 — Ongoing
Twice reviewed for PSJ, assessing research on atmospheric energy transport. I have reviewed for A&A on planet formation/disk chemistry, and for ApJ on atmospheric chemistry and radiation. I have also reviewed small grant applications for the UK government's research councils.

Co-supervision of MSc project: Obserable sulfur photochemistry linked to mantle redox

Summer 2025 — Summer 2026, Groningen
Sulfur can take on different redox states; its chemistry is closely linked to the redox state of its environment. This project investigated sulfur photochemistry during/following magma ocean stages, testing sulfur as a JWST-observable tracer of super-Earths interior conditions. Our student's work achieved the highest possible grade and is in-review for publication with Astronomy & Astrophysics.

Co-supervision of MSc project: Hadean ultraviolet context for origin of life

Summer 2025 — Summer 2026, Groningen and Berlin
The environment in which life emerged on Earth is unknown. Ultraviolet radiation is thought to enable the prebiotic chemical processes from which the first life arose. This project investigated the UV radiation exposure on the early Earth using planetary evolution simulations and atmosphere models.

Co-supervision of BSc project: tidal heating in the early Earth-Moon system

Spring — Summer 2025, Groningen
Following the formation of the Moon, substantial amounts of tidal heat energy may have been dissipated within Earth. This project theoretically tested how such heating influenced early Earth and the onset of habitable conditions. The student used the PROTEUS framework that was largely developed during my PhD, and later worked as a research assistant. This research was published in the Planetary Science Journal.

Two-years teaching undergraduate electromagnetism

Autumn 2023 — Summer 2025, Oxford
Teaching electromagnetism through lab and theory to undergraduate students at Oxford, during my PhD. The experiments involve calculating various physical constants and demonstrating known laws. Demonstrating in labs has taught me how to provide guidance and constructive feedback.

Co-supervision of BSc project on runaway greenhouse

Spring — Summer 2024, Groningen
This project aimed to test the robustness of the canonical runaway greenhouse effect, considering different secondary atmosphere compositions. We then worked to turn their thesis into a paper, published in ApJ. I guided the student through the research project, including the use of computer simulations and statistical analysis, and provided feedback and assessment of their dissertation at the end of the project.

Work experience

Summer internship on photonic cavities

June 2020 — August 2020, Exeter
Using proprietary FDTD software and open source data science libraries, I optimised photonic crystal cavities for the desired characteristics. My code was later applied to other photonic systems by the research group at Exeter.

Process engineering internship, Plessey Semiconductors

June 2019 — August 2019
I was directly involved in the development of cutting edge GaN micro-LED display technology with the Process Engineering team. During this work, I independently developed a novel computer-automated method to enable efficient colour conversion using quantum dot printing onto silicon wafers. Further experience included plasma etch tools and scanning electron microscopy.

Outreach and EDI (equality, diversity, inclusion)

EDI initatives at Cambridge

Spring 2026 — Ongoing
I am a member of the EDI committee at the Institute of Astronomy, Cambridge. My efforts focus on social and active initiatives which promote healthy work-life balance across the Institute, enabling rewarding connections between scientists and the professional staff.

Media interactions and public engagement

Summer 2026, media interviews and coverage
I collaborated with journalists to share my research characterising the magma ocean of a 'sulfur world' exoplanet (Nicholls+26b). This experience taught me how to clearly communicate and share research with diverse audiences, which included newspaper and radio platforms: The Guardian, Reuters, Financial Times, Scientific American, CBC 'Quirks' radio interview, 'Living on Earth' radio interview.

Primary school outreach programme

Autumn 2022, Oxford
Working directly within two primary schools in deprived communities of the greater Oxford area, I engaged Year 5 children with workshops on general physics. Introduced a class of 30 to broader physical concepts, and then tutored break-out groups to conduct hands-on experiments.

Funding and grants

Accolades

Non-academic

Programming and code development

Software I have developed

Languages